The present invention relates to a product and process for making gypsum panels. More particularly, the present invention relates to a gypsum panel with improved resistance to mold (also referred to as mildew).
Gypsum panels are well known building products which have been used for years. They are used primarily as an interior wall and ceiling product, but also to a certain extent as an exterior product. A slurry including calcium sulfate hemihydrate and water is used to form the core, and continuously deposited on a paper cover sheet moving beneath a mixer. A second paper cover sheet is applied thereover and the resultant assembly is formed into the shape of a panel. Calcium sulfate hemihydrate reacts with sufficient water to convert the hemihydrate into a matrix of interlocking calcium sulfate dihydrate crystals, causing it to set and to become firm. The continuous strip thus formed is conveyed on a belt until the calcined gypsum is set, and the strip is thereafter cut to form boards of desired length, which boards are conveyed through a drying kiln to remove excess moisture.
Fungi, such as mold, may grow in environments where four key elements are present. There must be mold spores present, nutrients for the fungi to metabolize and water. Temperature is also a critical parameter for fungi growth, but numerous mold species thrive at the temperatures required for human habitation so this is often considered a given for mold growth in buildings. Although various environments provide different amounts of each of these elements, water vapor and spores are constantly in the air around us. The spores require sufficient nutrients to be able to grow if they settle on a substrate where moisture is present.
While various nutrients are generally present in dust particles in surrounding air, starches also provide sufficient nutrition for mold growth and are often present in both the cover materials and the gypsum core of gypsum panels. In gypsum panels, starch is frequently used for a number of purposes. It is used to promote adhesion between the core and the covering material. The pressed paper commonly used to cover the panels is a source of starch and the cellulosic fibers provide nutrition for mold growth. Sugar is used to coat particles of calcium sulfate dihydrate, often used as a set accelerator in the calcined gypsum slurry. Other starches are also used to modify properties of the set gypsum composition. Thus, where gypsum board panels become wet and do not dry out readily, the use of starches in covering and core materials provides a medium suitable for possible growth of mold spores. Gypsum board panels, even if not specially treated to make them mold-resistant, still will not usually experience mold growth problems in interior building applications or in other applications where they are likely to be kept dry or to dry out readily after becoming wet.
However, there are some applications where gypsum board is desirable for its fire resistance, but where it may become wet and not readily dry out. In high-rise buildings, for example, elevator shafts are built before the building is enclosed. Thick gypsum panels, such as Sheetrock® brand Gypsum Liner Panels by USG Corp., Chicago, Ill., are used to line the elevator shafts to provide fire resistance. The shaft wall may be exposed to rain during building construction, and may not have the opportunity to dry thoroughly before the building is closed in. Panels used in this environment, and other environments where mold growth is possible, are subject to improvement by increasing the resistance of the panels to the growth of molds.
Gypsum panels are known that have utilized pressed paper coverings treated with a fungicide. Treated paper is ineffective to control mold growth for a number of reasons. Many fungicides do not retain their efficacy through the process of drying the panels in a kiln due to the high temperatures. Water used in the manufacturing of gypsum panels may contain mold spores, providing a source of spores from both the air and the set gypsum. Per environmental regulations, there is a limit to the concentration of fungicide that can be present on the surface of the paper, and it appears that that concentration is not sufficient to protect both the paper and the set gypsum core.
Attempts have been made to add fungicides to the gypsum slurry, resulting in different problems. Water-soluble fungicides tend to migrate with the water during the drying process, depositing on the covering when the water evaporates. In addition to leaving the core unprotected, the paper covering may have a concentration of fungicide that is too high to meet environmental regulations. Fungicides that are insoluble are difficult to disperse in the aqueous slurry and provide no protection for the covering material. Chemicals added directly to the gypsum slurry can also have detrimental effects on the properties of the set gypsum product. When boric acid, a known fungicide, was added to a slurry in sufficient quantity to greatly inhibit mold growth, the panels were so brittle that they cracked and chipped as they moved along the rollers from the kiln.
Another technique of protecting a gypsum board is using a two-step process of covering a fungicide-containing core slurry with a treated face paper. In addition to many of the problems discussed above, use of a two-step process is more expensive than a single step process. Thus, although many fungicides are well known, this particular application poses unique problems in finding a fungicide that inhibits mold growth in both the covering and the core of gypsum board panels in a cost effective manner.
Pyrithione salts are well-known antimicrobial additives for coating applications. They are available commercially as sodium OMADINE® or zinc OMADINE, manufactured by Arch Chemicals, Inc. of Norwalk, Conn., or they can be made according to the process of U.S. Pat. No. 3,159,640, herein incorporated by reference. The prior art teaches only that these salts are useful in their wet state as preservatives or as short-term antimicrobial agents in dry, thin-film applications such as paints, adhesives, caulks and sealants. U.S. Pat. No. 5,939,203 discloses that joint compounds and patching compounds are suitable base media for use with pyrithione salts in coating compositions. Joint or patching compounds are thinly spread over joints between or imperfections in gypsum board panels, forming a thin film. The use of sodium pyrithione in these compounds would act as a wet state preservative for ready mixed products and would inhibit microbial growth on the dry film of the product.